OFFSET
1,1
EXAMPLE
1*(2^5)-1 = 31, 31 is prime, so a(1) = 2.
2*((2*3)^5)-1 = 15551, 15551 is prime, so a(2) = 3.
3*((2*3)^5)-1 = 23327, 23327 is prime, so a(3) = 3.
MATHEMATICA
a[n_] := If[n > 1 && IntegerQ[Surd[n, 5]], 0, Module[{p = pr = 2}, While[! PrimeQ[n*pr^5 - 1], p = NextPrime[p]; pr *= p]; p]]; Array[a, 100] (* Amiram Eldar, Sep 11 2021 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Pierre CAMI, May 25 2006
EXTENSIONS
Data corrected by Amiram Eldar, Sep 11 2021
STATUS
approved